Evidence for a Spoken Word Lexicon in the Auditory Ventral Stream.

The existence of a neural representation for whole words (i.e., a lexicon) is a common feature of many models of speech processing. Prior studies have provided evidence for a visual lexicon containing representations of whole written words in an area of the ventral visual stream known as the visual word form area. Similar experimental support for an auditory lexicon containing representations of spoken words has yet to be shown. Using functional magnetic resonance imaging rapid adaptation techniques, we provide evidence for an auditory lexicon in the auditory word form area in the human left anterior superior temporal gyrus that contains representations highly selective for individual spoken words. Furthermore, we show that familiarization with novel auditory words sharpens the selectivity of their representations in the auditory word form area. These findings reveal strong parallels in how the brain represents written and spoken words, showing convergent processing strategies across modalities in the visual and auditory ventral streams.

From shape to meaning: Evidence for multiple fast feedforward hierarchies of concept processing in the human brain.

A number of fMRI studies have provided support for the existence of multiple concept representations in areas of the brain such as the anterior temporal lobe (ATL) and inferior parietal lobule (IPL). However, the interaction among different conceptual representations remains unclear. To better understand the dynamics of how the brain extracts meaning from sensory stimuli, we conducted a human high-density electroencephalography (EEG) study in which we first trained participants to associate pseudowords with various animal and tool concepts. After training, multivariate pattern classification of EEG signals in sensor and source space revealed the representation of both animal and tool concepts in the left ATL and tool concepts within the left IPL within 250 â€‹ms. Finally, we used Granger Causality analyses to show that orthography-selective sensors directly modulated activity in the parietal-tool selective cluster. Together, our results provide evidence for distinct but parallel "perceptual-to-conceptual" feedforward hierarchies in the brain.

An fMRI-adaptation study of phonological and orthographic selectivity to written words in adults with poor reading skills.

Typical readers rely on two brain pathways for word processing in the left hemisphere: temporo-parietal cortex (TPC) and inferior frontal cortex (IFC), thought to subserve phonological decoding, and occipito-temporal cortex (OTC), including the "visual word form area" (VWFA), thought to subserve orthographic processing. How these regions are affected in developmental dyslexia has been a topic of intense research. We employed fMRI rapid adaptation (fMRI-RA) in adults with low reading skills to examine in independently-defined functional regions of interest (ROIs) phonological selectivity to written words in left TPC and IFC, and to orthographic selectivity to written words in OTC. Consistent with the phonological deficit hypothesis of dyslexia, we found responsivity but not selectivity to phonology, as accessed by written words, in the posterior superior temporal gyrus (pSTG) of the TPC. On the other hand, we found orthographic selectivity in the VWFA of the OTC. We also found selectivity to orthographic and not phonological processing in the IFG, a finding previously reported for typical readers. Together our results demonstrate that in adults with poor reading skills, selectivity to phonology is compromised in pSTG, while selectivity to orthography in the VWFA remains unaffected at this level of processing.

Orthographic and phonological selectivity across the reading system in deaf skilled readers.

People who are born deaf often have difficulty learning to read. Recently, several studies have examined the neural substrates involved in reading in deaf people and found a left lateralized reading system similar to hearing people involving temporo-parietal, inferior frontal, and ventral occipito-temporal cortices. Previous studies in typical hearing readers show that within this reading network there are separate regions that specialize in processing orthography and phonology. We used fMRI rapid adaptation in deaf adults who were skilled readers to examine neural selectivity in three functional ROIs in the left hemisphere: temporoparietal cortex (TPC), inferior frontal gyrus (IFG), and the visual word form area (VWFA). Results show that in deaf skilled readers, the left VWFA showed selectivity for orthography similar to what has been reported for hearing readers, the TPC showed less sensitivity to phonology than previously reported for hearing readers using the same paradigm, and the IFG showed selectivity to orthography, but not phonology (similar to what has been reported previously for hearing readers). These results provide evidence that while skilled deaf readers demonstrate coarsely tuned phonological representations in the TPC, they develop finely tuned representations for the orthography of written words in the VWFA and IFG. This result suggests that phonological tuning in the TPC may have little impact on the neural network associated with skilled reading for deaf adults.

Evidence for rapid localist plasticity in the ventral visual stream: The example of words.

Our recent work has shown that the Visual Word Form Area (VWFA) in left occipitotemporal cortex contains an orthographic lexicon based on neuronal representations highly selective for individual written real words (RW) and that learning novel words selectively increases neural specificity in the VWFA. But, how quickly does this change in neural tuning occur and how much training is required for new words to be codified in the VWFA? Here we present evidence that plasticity in the VWFA from broad to tight tuning can be obtained in a short time span, with no explicit training, and with comparatively few exposures, further strengthening the case for a highly plastic visual lexicon in the VWFA and for localist representations in the visual processing hierarchy.

Multivariate Pattern Analysis Reveals Category-Related Organization of Semantic Representations in Anterior Temporal Cortex.

UNLABELLED: The neural substrates of semantic representation have been the subject of much controversy. The study of semantic representations is complicated by difficulty in disentangling perceptual and semantic influences on neural activity, as well as in identifying stimulus-driven, "bottom-up" semantic selectivity unconfounded by top-down task-related modulations. To address these challenges, we trained human subjects to associate pseudowords (TPWs) with various animal and tool categories. To decode semantic representations of these TPWs, we used multivariate pattern classification of fMRI data acquired while subjects performed a semantic oddball detection task. Crucially, the classifier was trained and tested on disjoint sets of TPWs, so that the classifier had to use the semantic information from the training set to correctly classify the test set. Animal and tool TPWs were successfully decoded based on fMRI activity in spatially distinct subregions of the left medial anterior temporal lobe (LATL). In addition, tools (but not animals) were successfully decoded from activity in the left inferior parietal lobule. The tool-selective LATL subregion showed greater functional connectivity with left inferior parietal lobule and ventral premotor cortex, indicating that each LATL subregion exhibits distinct patterns of connectivity. Our findings demonstrate category-selective organization of semantic representations in LATL into spatially distinct subregions, continuing the lateral-medial segregation of activation in posterior temporal cortex previously observed in response to images of animals and tools, respectively. Together, our results provide evidence for segregation of processing hierarchies for different classes of objects and the existence of multiple, category-specific semantic networks in the brain. SIGNIFICANCE STATEMENT: The location and specificity of semantic representations in the brain are still widely debated. We trained human participants to associate specific pseudowords with various animal and tool categories, and used multivariate pattern classification of fMRI data to decode the semantic representations of the trained pseudowords. We found that: (1) animal and tool information was organized in category-selective subregions of medial left anterior temporal lobe (LATL); (2) tools, but not animals, were encoded in left inferior parietal lobe; and (3) LATL subregions exhibited distinct patterns of functional connectivity with category-related regions across cortex. Our findings suggest that semantic knowledge in LATL is organized in category-related subregions, providing evidence for the existence of multiple, category-specific semantic representations in the brain.

Uncovering phonological and orthographic selectivity across the reading network using fMRI-RA.

Reading has been shown to rely on a dorsal brain circuit involving the temporoparietal cortex (TPC) for grapheme-to-phoneme conversion of novel words (Pugh et al., 2001), and a ventral stream involving left occipitotemporal cortex (OTC) (in particular in the so-called "visual word form area", VWFA) for visual identification of familiar words. In addition, portions of the inferior frontal cortex (IFC) have been posited to be an output of the dorsal reading pathway involved in phonology. While this dorsal versus ventral dichotomy for phonological and orthographic processing of words is widely accepted, it is not known if these brain areas are actually strictly sensitive to orthographic or phonological information. Using an fMRI rapid adaptation technique we probed the selectivity of the TPC, OTC, and IFC to orthographic and phonological features during single word reading. We found in two independent experiments using different task conditions in adult normal readers, that the TPC is exclusively sensitive to phonology and the VWFA in the OTC is exclusively sensitive to orthography. The dorsal IFC (BA 44), however, showed orthographic but not phonological selectivity. These results support the theory that reading involves a specific phonological-based temporoparietal region and a specific orthographic-based ventral occipitotemporal region. The dorsal IFC, however, was not sensitive to phonological processing, suggesting a more complex role for this region.

Adding words to the brain's visual dictionary: novel word learning selectively sharpens orthographic representations in the VWFA.

The nature of orthographic representations in the human brain is still subject of much debate. Recent reports have claimed that the visual word form area (VWFA) in left occipitotemporal cortex contains an orthographic lexicon based on neuronal representations highly selective for individual written real words (RWs). This theory predicts that learning novel words should selectively increase neural specificity for these words in the VWFA. We trained subjects to recognize novel pseudowords (PWs) and used fMRI rapid adaptation to compare neural selectivity with RWs, untrained PWs (UTPWs), and trained PWs (TPWs). Before training, PWs elicited broadly tuned responses, whereas responses to RWs indicated tight tuning. After training, TPW responses resembled those of RWs, whereas UTPWs continued to show broad tuning. This change in selectivity was specific to the VWFA. Therefore, word learning appears to selectively increase neuronal specificity for the new words in the VWFA, thereby adding these words to the brain's visual dictionary.

Individual variability in location impacts orthographic selectivity in the "visual word form area".

Strong evidence exists for a key role of the human ventral occipitotemporal cortex (vOT) in reading, yet there have been conflicting reports about the specificity of this area in orthographic versus nonorthographic processing. We suggest that the inconsistencies in the literature can be explained by the method used to identify regions that respond to words. Here we provide evidence that the "visual word form area" (VWFA) shows word selectivity when identified at the individual subject level, but that intersubject variability in the location and size of the VWFA causes this selectivity to be washed out if defining the VWFA at the group level or based on coordinates from the literature. Our findings confirm the existence of a word-selective region in vOT while providing an explanation for why other studies have found a lack of word specificity in vOT.

Overt use of a tactile-kinesthetic strategy shifts to covert processing in rehabilitation of letter-by-letter reading.

BACKGROUND: Letter-by-letter readers identify each letter of the word they are reading serially in left to right order before recognizing the word. When their letter naming is also impaired, letter-by-letter reading is inaccurate and can render even single word reading very poor. Tactile and/or kinesthetic strategies have been reported to improve reading in these patients, but only under certain conditions or for a limited set of stimuli. AIMS: The primary aim of the current study was to determine whether a tactile/kinesthetic treatment could significantly improve reading specifically under normal reading conditions, i.e. reading untrained words presented in free vision and read without overt use of the strategy. METHODS #ENTITYSTARTX00026; PROCEDURES: Three chronic letter-by-letter readers participated in a tactile/kinesthetic treatment aimed at first improving letter naming accuracy (phase 1) and then letter-by-letter reading speed (phase 2). In a multiple case series design, accuracy and speed of reading untrained words without overt use of the trained tactile/kinesthetic strategy was assessed before phase 1, after phase 1 and again after phase 2. OUTCOMES #ENTITYSTARTX00026; RESULTS: All three patients significantly improved both their speed and accuracy reading untrained words without overt use of the trained tactile/kinesthetic strategy. All three patients required the additional practice in phase 2 to achieve significant improvement. Treatment did not target sentence level reading, yet two of the three patients became so adept that they could read entire sentences. CONCLUSIONS: This study replicates previous findings on the efficacy of tactile/kinesthetic treatment for letter-by-letter readers with poor letter naming. It further demonstrates that this treatment can alter cognitive processing such that words never specifically trained can be read in free vision without overtly using the trained strategy. The data suggest that an important element in achieving this level of generalization is continuing training beyond the point of initial mastery (i.e. accurate letter naming).

Evidence for highly selective neuronal tuning to whole words in the "visual word form area".

Theories of reading have posited the existence of a neural representation coding for whole real words (i.e., an orthographic lexicon), but experimental support for such a representation has proved elusive. Using fMRI rapid adaptation techniques, we provide evidence that the human left ventral occipitotemporal cortex (specifically the "visual word form area," VWFA) contains a representation based on neurons highly selective for individual real words, in contrast to current theories that posit a sublexical representation in the VWFA.